使用關係數據庫

Question

我有三個表格Business，Users和Subscriptions。目標是能夠獲得與用戶相關的業務名稱。結果應該是用戶在訂閱簽署了表的

列表

Schema |  Name  | Type | 
--------+-------------------+------- 
public | businesses  | table 
public | subscriptions  | table 
public | users    | table 

---

業務表

Column |   Type               
------------+----------------------------- 
id   | integer      
name  | character varying(255)  
created_at | timestamp without time zone 
updated_at | timestamp without time zone 

商業模式

class Business < ActiveRecord::Base 

end 

企業名稱的數組

---

訂閱表

Column |   Type         
-------------+----------------------------- 
id   | integer      
user_id  | integer      
business_id | integer      
created_at | timestamp without time zone 
updated_at | timestamp without time zone 

訂閱模型

class Subscription < ActiveRecord::Base 
    belongs_to :user 
    belongs_to :business 
end 

---

用戶表

 Column  |   Type         
-----------------+----------------------------- 
id    | integer     
username  | character varying(255)  
password_digest | character varying(255)  
email   | character varying(255)  
created_at  | timestamp without time zone 
updated_at  | timestamp without time zone 

用戶模型

class User < ActiveRecord::Base 
     has_secure_password 
     has_many :subscriptions 
    end 

---

Rails的控制檯

u = User.find(1) 
u.subscriptions.businesses.name 

Response: undefined method `business' 

Answer 1

而不是做一個額外的查詢每一個訂閱（如在阿里的答案），你可以使用Rails的只是一個查詢做到這一點內置includes方法：

subscriptions = u.subscriptions.includes(:business) 

這將返回所有訂閱及其相關企業的關係。現在，你可以得到企業的名稱，而不做其他查詢：

business_names = subscriptions.map {|sub| sub.business.name } 

我認爲你也可以做到這一點的一個步驟中使用pluck：

business_names = u.subscriptions.includes(:business).pluck("businesses.name") 

我不能肯定pluck戲劇不錯，includes，不過值得一試。

Answer 2

，因爲用戶有許多訂閱，調用u.subscriptions返回訂閱的數組，所以你通過數組需要循環，並呼籲每個記錄business關聯。你可以實現你需要使用的是什麼：

business_names = u.subscriptions.map {|subscription| subscription.business.name }